The determination of neutral winds and ion drifts in low-Earth orbits may require measurements of the angular and energy distributions of the flux of neutrals and ions entering a satellite from the ram direction. The ram direction may be the direction of the average or total velocity T of the air stream passing aperture 10 as shown in FIG. 1. The lower portion of FIG. 1 shows a simulation of the angular distribution of the flux F (atomic oxygen is shown) entering an aperture 10 of a spectrometer. The angular distribution of the flux F is shown with respect to two mutually perpendicular axes tau and theta. The angular distribution of the flux F may be generally confined within a cone roughly centered along the ram axis A (direction of satellite movement) and generally azimuthally symmetric. The orientation of the aperture 10 may not necessarily coincide with A.
The average total velocity T of the air is the sum of the satellite velocity S and the wind vector W. The magnitude and direction of the neutral wind (or ion drift) vector W determines the location of the maximum in the angular distribution of the flux F. Knowledge of the angle of maximum flux with respect to the satellite's coordinates, and the satellite's pointing with respect to S, may be used to determine the wind (or ion drift) vector W.
Spectrometers may detect the angular and energy distributions of neutral atoms/molecules and ions in two mutually perpendicular planes. A small-deflection energy analyzer (SDEA) is described in “The Gated Electrostatic Mass Spectrometer (GEMS): Definition and Preliminary Results”; F. A. Herrero, H. H. Jones, J. G. Lee; Journal Of American Society for Mass Spectrometry 2008, 19, pp. 1384-1394, Jul. 18, 2008, which is expressly incorporated by reference in its entirety herein. On page 1388 of the cited article, it is noted that the exit slit may be laid out in a circular arc spanned by an angle, such as theta or tau, so that all trajectories have the same length along the SDEA. For measurements along two mutually perpendicular planes, such as the planes spanned by the axes tau and theta of FIG. 1, separate spectrometers and ion sources may be used for the measurements along each of the two mutually perpendicular planes. Each ion source may require electric power and may occupy a certain volume. A need exists for a spectrometer that uses less electric power and occupies less space.